This invention relates generally to lasers and, more specifically, to semiconductor lasers operated in an external cavity.
A laser is a device which has the ability to produce monochromatic, coherent light through the stimulated emission of photons from atoms, molecules or ions of an active gain medium which have typically been excited from a ground state to a higher energy level by an input of energy. Such a device contains an optical cavity or resonator which is defined by highly reflecting surfaces which form a closed round trip path for light. The active gain medium is contained within the optical cavity.
If a population inversion is created by excitation of the active medium, the spontaneous emission of a photon from an excited atom, molecule or ion undergoing transition to a lower energy state can stimulate the emission of photons of substantially identical energy from other excited atoms, molecules or ions. As a consequence, the initial photon creates a cascade of photons between the reflecting surfaces of the optical cavity which are of substantially identical energy and exactly in phase. This multiplication effect causes light inside the cavity to undergo gain, which, along with the feedback provided by the resonator, constitutes a laser oscillator. A portion of this cascade of photons is then discharged out of the optical cavity, for example, by transmission through one or more of the reflecting surfaces of the cavity. These discharged photons constitute the laser output.
Excitation of the active medium of a laser can be accomplished by a variety of methods. However, the most common methods are optical pumping, use of an electrical discharge, and the passage of an electric current through the p-n junction of a semiconductor laser.
Semiconductor lasers contain a p-n junction which forms a diode, and this junction functions as the active medium of the laser. Such devices, which are also referred to as diode lasers, are typically constructed from materials such as gallium arsenide and aluminum gallium arsenide alloys. The efficiency of such lasers in converting electrical power to output radiation is relatively high and, for example, can be in excess of 40 percent.
In recent years, improvements in laser performance have resulted from the fabrication of so-called "superlattice" and quantum-well (QW) structures. A lattice is formed of alternate layers of thin (5-40 nm) epitaxial films of semiconductor material of different band gap to form a plurality of heterojunction interfaces. A thin film of narrow band gap material, e.g., GaAs, is sandwiched between thin films of wider band gap material, e.g., Al.sub.x Ga.sub.(1-x) As to form potential wells. Such wells restrict or limit carrier/electron movement to two dimensions. Such a multilayer is referred to as a two-dimensional quantum-well, or simply a quantum-well. In the example given, the electron motion is restricted in the direction perpendicular to the heterojunction interfaces, while the electrons are free to move in the other two directions.
Quantum-well lasers generally exhibit lower threshold than bulk crystal heterostructures. Devices are usually formed of multiple layers of quantum-wells and when the barrier thickness between quantum-wells is reduced, so that well wave functions couple, a "superlattice" is formed.
Generally, the materials of choice for superlattice and/or quantum-well structures are the Group III and V elements and, in particular, GaAs and Al alloys thereof. These materials are closely lattice matched, yet the difference in the band gaps of the GaAs versus the alloy Al.sub.x Ga.sub.(1-x) As can vary at room temperature from 0 eV to as much as 0.75 eV (as x increases from 0 to 1). The first property simplifies heterostructure fabrication, while the second property makes fabrication of quantum-wells possible.